I gave mine a decade's endurance, on 5% survey speed, and it was enough that they'd roam through a system or two each before they'd have to come back in. Later on, with phased array survey sensors, they could plow through 5 or 6 systems solo in that same time span.
Made them 10,000 tons initially, shrank as tech improved, one of each sensor, own jump drive as soon as it could be made to fit within 10,000 tons. Send them to a system, and forget they exist for a few years of game time.